Revision as of 14:07, 21 August 2013

An orphan instance is a type class instance for class C and type T which is neither defined in the module where C is defined nor in the module where T is defined.

Type class instances are special in that they don't have a name and cannot be imported explicitly. This also means that they cannot be excluded explicitly. All instances defined in a module A are imported automatically when importing A, or importing any module that imports A, directly or indirectly.

Say you want to define an alternative instance to an existing instance. This is a bad thing, since if two instances for the same class/type pair are in scope, then you cannot describe in Haskell 98 which instance to use. If you want to use multiple instances for the same class/type, you have to ensure that they are never imported together in a module somewhen. It is almost impossible to assert that, or put differently, it would reduce the composability of libraries considerably.

Actually, non-orphan instances can avoid definition of multiple instances. For defining an instance you have to import the class and the type and then you will automatically have the according non-orphan instances imported, too. If you want to define a new instance then the compiler will reject it immediately.

1 When Orphan Instances can be useful

It is worth noting that Orphan Instances can be viewed as a mechanism for writing modules of code with a fixed typed interface, but parameterized over the choice of implementation. In this case, Orphan Instances act as a sort of plugin architecture for providing alternative implementations with a uniform interface.